This invention relates generally to solid propellant rocket engines, and, more particularly, to an improved rocket nozzle system for use therewith.
Rocket engines are generally classified as either liquid propellant engines, solid propellant engines or hybrid engines (i.e., a combination of liquid and solid propellants). Although the instant invention is capable of incorporation within any of the above-mentioned types of rocket engines, its main utility has been found in the solid propellant rocket engine. Therefore, a brief analysis of the solid propellant rocket engine is set forth hereinbelow.
In a solid propellant rocket engine the propellant is contained in solid form within the combustion chamber. In addition to the combustion chamber, the other basic elements of the solid propellant engine, as in other rocket engines, are the nozzle and igniter. The solid propellants usually have a cake-like appearance (grain) and burn on their exposed surfaces to form hot exhaust gases that exhaust through the nozzle.
The solid-propellant grain contains all material necessary for sustaining combustion. It can be a mixture of several chemicals, for example, a mixture of ammonium perchlorate in a matrix of organic fuel such as rubber, or it can be a homogeneous charge of special oxidizing organic chemicals, such as nitrocellulose or nitroglycerine. Once the propellant is ignited, the grain burns smoothly on its exposed surface in a direction normal to the burning surface.
The chamber confines the propellant grain and the reaction products of combustion. It is generally made of high-strength material, such as alloy steel or fiber-reinforced plastics, and is often protected on its exposed surfaces from excessive heating by internal insulating layers made of either a slow-burning rubbery or plastic liner, or sometimes a ceramic insulator. This liner, which is a thin layer next to the wall of the case, serves not only as a thermal insulator but also as a means for obtaining good adhesion or bonding of the grain to the case.
Combustion chambers are usually cylindrical in shape, with elliptical or spherical ends. Diaphragms, blowout closures, or other safety provisions have been used to prevent overpressurization of the chamber. In some designs the burst diaphragms are intentionally activated to stop the application of thrust at a predetermined time. If the grain is not case-bonded, the chamber ordinarily has means for holding the grain in place and positioning it, so that the acceleration of the vehicle will not force it into the nozzle. In these designs one end of the chamber is usually detachable to permit loading of the grain. In case-bonded grains the assembly opening is usually smaller than the maximum diameter of the unit, just large enough to permit the insertion and subsequent withdrawl of the mandrel. In addition, the chamber usually has provisions for mounting to the vehicle, and for sealing against moisture, which would cause deterioration of certain grain chemicals.
The most severe heating of the hardware takes place at the exhaust nozzle. Here high-velocity gas at high-combustion temperature oxidizes, softens, wears, and erodes the nozzle material. If the nozzle throat is eroded unsymmetrically, then the thrust vector direction may shift and cause the flight to become erratic. For this reason it if often desirable to put special heat-resistant ceramic or graphite inserts into the nozzle throat region to minimize unsymmetrical enlargement of the nozzle area. Ceramic nozzles have been successful for long periods of time: however, their effectiveness for high-temperature propellants seems to be limited to approximately 2 min. It is therefore essential for optimum solid propellant rocket engine operation to provide a rocket nozzle which is capable of effective economical and reliable high temperature operation.